It seems what you are referring to would be "contractual" sponsorship.
Yes absolutely.
Remember that a what a girl really wants is a guy who looks after her for life not just from Christmas (or while she looks good).
Now if you offer her sponsorship which is temprorary you are openly admitting that your relationship is temporary and that you will probably dump her later. If you do that - fine... but... dont expect her to remain faithful to you when she is really looking for a lifetime commitment. She will always be open to a better offer or alternative offer.
Now of course some guys like their girl so much they marry her - inherently making that lifetime commitment financially (and usually sexually). Now with honest sponsorship you are basically saying I will support you financially (that is my sacrifice to you) so long as you remain faithful sexually to me (that is your sacrifice to me).
Now I am not saying that this is the best solution for everyone but it is an 'honest solution' for those that want a strong friendship with a TG.
I am sure a lot of guys dont really give a toss whether their TG or BG is faithful or not (in fact as the BGs are hookers they know they are not). That's fine too. Your are renting her for a couple of hours or a couple of weeks and once that contractual arrangement is complete you go your seperate ways.